The present invention relates to a method for fabricating a semiconductor device such as a short wavelength light-emitting device, particularly a GaN-based semiconductor laser device, or the like, which is expected to be used in the field of optical information processing, etc.
Among the group III-V compound semiconductors, nitride semiconductors containing nitrogen (N) as the group V element are seen to be promising materials of a short wavelength light-emitting device for their relatively large band gap. Among others, so-called nitride semiconductors made of a gallium nitride-based compound semiconductor, e.g., a GaN-based semiconductor of AlxGayInzN (where 0xe2x89xa6x, y, zxe2x89xa61, x+y+z=1), have been actively researched, and blue or green light-emitting diode devices (LEDs) have been put into practical use. In order to increase the capacity of an optical disk apparatus, there is a strong demand for a semiconductor laser device having a 400 nm band oscillation wavelength. To this end, a semiconductor laser device using a GaN-based semiconductor has been attracting public attention, and the current state of the art of such a semiconductor laser device is close to a practical level.
A conventional GaN-based semiconductor laser device will now be described with reference to the accompanying drawings.
FIG. 7 is a cross-sectional view illustrating a conventional GaN-based semiconductor laser device which has been confirmed to produce laser oscillation. As illustrated in FIG. 7, a buffer layer 102 made of GaN for reducing the lattice mismatch between a substrate 101 and a GaN-based semiconductor grown on the substrate 101, an n-type contact layer 103 made of n-type GaN which includes a device formation region and an n-side electrode formation region, a first cladding layer 104 made of n-type Al0.07Ga0.93N which is formed in the device formation region of the n-type contact layer 103, a first optical guide layer 105 made of n-type GaN, a multiple quantum well active layer 106 made of a stack of Ga1xe2x88x92xInxN/Ga1xe2x88x92yInyN (0 less than y less than x less than 1), a second optical guide layer 107 made of p-type GaN, a second cladding layer 108 made of p-type Al0.07Ga0.93N, and a p-type contact layer 109 made of p-type GaN, are formed in this order on the substrate 101 made of sapphire by using a metal-organic vapor phase epitaxy (MOVPE) method.
A ridge stripe portion 108a whose width is about 3 xcexcm to about 10 xcexcm is formed on the second cladding layer 108, and the p-type contact layer 109 is formed on the ridge stripe portion 108a. The side region of the p-type contact layer 109 and the ridge stripe portion 108a of the second cladding layer 108 and the side surfaces of the epitaxial layers are covered with an insulation film 110.
A p-side electrode 111 made of a stack of Ni/Au, for example, is formed on the insulation film 110 including the p-type contact layer 109 so as to be in contact with the p-type contact layer 109, and an n-side electrode 112 made of a stack of Ti/Al, for example, is formed in the n-side electrode formation region of the n-type contact layer 103.
In a semiconductor laser device having such a structure, when the n-side electrode 112 is grounded and a voltage is applied to the p-side electrode 111, holes and electrons are injected into the multiple quantum well active layer 106 from the p-side electrode 111 side and from the n-side electrode 112 side, respectively. The injected holes and electrons produce an optical gain in the multiple quantum well active layer 106, thereby causing laser oscillation with an oscillation wavelength of about 400 nm. Note, however, that the oscillation wavelength varies depending upon the composition or thickness of the stack of Ga1xe2x88x92xInxN/Ga1xe2x88x92yInyN forming the multiple quantum well active layer 106. At present, there has been realized a semiconductor laser device of this structure capable of continuous-wave oscillation at room temperature or higher.
It is disclosed that in order to grow a GaN-based semiconductor containing In, i.e., a semiconductor made of GaInN, by using an MOVPE method, it is preferred to set the crystal growth temperature to about 800xc2x0 C. and to use a nitrogen (N2) gas as a carrier gas (Applied Physics Letters, vol. 59, p. 2251, 1991). On the other hand, for a semiconductor layer containing no In such as the cladding layer 104, 108 made of Al0.07Ga0.93N, the optical guide layer 105, 107 made of GaN, or the like, it is typical that the growth temperature is as high as 1000xc2x0 C. or higher and a hydrogen (H2) gas is used as the carrier gas. The series of growth processes are described in detail in, for example, Japanese Laid-Open Patent Publication No. 6-196757 or Japanese Laid-Open Patent Publication No. 6-177423.
The outline of the processes will be described below.
First, the substrate 101 is held in a reaction chamber, and a heat treatment is performed by increasing the substrate temperature to 1050xc2x0 C. while introducing an H2 gas. Next, after the substrate temperature is decreased to 510xc2x0 C., ammonia (NH3) and trimethylgallium (TMG), which are reaction gases, are introduced onto the substrate 101, thereby growing the buffer layer 102 made of GaN.
Then, the introduction of TMG is stopped and the substrate temperature is increased to 1030xc2x0 C., and TMG and monosilane (SiH4) are introduced, with an H2 gas as a carrier gas, thereby growing the n-type contact layer 103 made of n-type GaN, after which trimethylaluminum (TMA) is added as a group III material gas containing Al, thereby successively growing the first cladding layer 104 made of n-type Al0.07Ga0.93N.
Then, the supply of the material gasses is stopped, and the substrate temperature is decreased to 800xc2x0 C. Then, the carrier gas is switched from an H2 gas to an N2 gas, and TMG and trimethylindium (TMI) are introduced as group III material gases, thereby forming the multiple quantum well active layer 106 made of a stack of Ga1xe2x88x92xInxN/Ga1xe2x88x92yInyN.
Then, the supply of the group III material gas is stopped, and the substrate temperature is increased again to 1020xc2x0 C., while the carrier gas is switched back from an N2 gas to an H2 gas, and TMG, TMA and cyclopentadienylmagnesium (Cp2Mg), or the like, as a p-type dopant are introduced, thereby successively growing the second optical guide layer 107 made of p-type GaN, the second cladding layer 108 made of p-type Al0.07Ga0.93N and the p-type contact layer 109 made of p-type GaN.
In the heating step after the formation of the active layer, a protection layer made of GaN (Japanese Laid-Open Patent Publication No. 9-186363) or a protection layer made of Al0.2Ga0.8N (e.g., Japanese Journal of Applied physics, Vol. 35, p. L74, 1996) may be formed in order to prevent re-evaporation of In from the multiple quantum well active layer 106.
However, in the conventional GaN-based semiconductor laser device as described above, a deterioration in the quality of the n-type contact layer 103 made of n-type GaN or the first cladding layer 104 made of n-type Al0.07Ga0.93N (note that the Al content is 0.1 in Japanese Laid-Open Patent Publication No. 6-196757 or Japanese Laid-Open Patent Publication No. 6-177423) to be the underlying layer for growing the multiple quantum well active layer 106 thereon causes a deterioration in the crystal quality of the multiple quantum well active layer 106 to be grown thereon, thereby leading to problems such as a deterioration in the light-emitting efficiency or an increase in the threshold current in the light-emitting diode device or the semiconductor laser device.
The present invention has been made in view of the above-described problems in the prior art, and has an object to improve the crystal quality of an active region or a peripheral region of the active region so as to realize a nitride semiconductor device having desirable operating characteristics.
In order to achieve the above-described object, according to the first invention, there is provided a flatness maintenance layer made of a nitride semiconductor for maintaining the surface flatness of a semiconductor layer by suppressing evaporation of the constituent atoms of the semiconductor layer when the semiconductor layer provided between a substrate and an active layer is formed through crystal growth. Particularly, the flatness maintenance layer is provided between a first semiconductor layer which is grown at a relatively high temperature and a second semiconductor layer which is grown at a temperature lower than that for the first semiconductor layer.
According to the second invention, a group III material gas is switched from one to another between the step of forming the first semiconductor layer and the step of forming the second semiconductor layer, wherein the ambience around the substrate is adjusted to be an ambience having a pressure of 1 atm or higher and containing nitrogen element, e.g., an ammonia gas, or the like, when the introduction of the group III material gas is temporarily stopped or the carrier gas is changed.
The present inventors have conducted various studies for reasons why a GaN-based semiconductor device has a low crystallinity in and around the active region. As a result, the results as set forth below have been obtained.
As a characteristic of the process of growing a type of material containing GaN, the carrier gas carrying the material gas may differ between a layer containing In, e.g., the multiple quantum well active layer 106 illustrated in FIG. 7, and a layer containing no In, e.g., the cladding layer 104 or the guide layer 105 illustrated in FIG. 7. Typically, a nitrogen gas is used as the carrier gas when growing the layer containing In, and a hydrogen gas is used for the layer containing no In. Thus, in order to fabricate a device by providing a stack of multiple layer films as in a semiconductor laser device, it is necessary to switch a carrier gas from one to another for each layer.
Moreover, for the multiple quantum well active layer 106 containing In and the cladding layer 104, or the like, containing no In, it is necessary to reduce the growth temperature for the layer containing In to be lower than that for the layer containing no In in order to suppress the vapor pressure of In.
When the carrier gas is switched from one to another, the supply of the group III material gas is stopped, whereby a so-called equilibrium state is provided where no additional crystal grows on the substrate 101. In the equilibrium state, elimination (re-evaporation) of the constituent atoms from the grown semiconductor layer occurs in a high temperature state such that the substrate temperature is as high as about 1000xc2x0 C. or under a pressure condition of 1 atm or less. Thus, the crystal quality deteriorates by the elimination of the constituent atoms from the growth interface of the semiconductor crystal in the equilibrium state when the material gas is switched from one to another.
To address the problem, the present inventors have discovered that if a flatness maintenance layer made of a nitride semiconductor containing Al is provided on the surface of the underlying layer, as the first solution for preventing the elimination of the constituent atoms from the growth interface of the GaN-based semiconductor crystal, then the crystal quality of the active layer which is grown on the underlying layer does not deteriorate.
Moreover, it has been discovered that if the ambience around the underlying layer is adjusted to be a pressurized gas ambience containing nitrogen element, as the second solution for preventing the elimination of the constituent atoms from the growth interface of the semiconductor crystal in the equilibrium state when the material gas is switched from one to another, then the crystal quality of the active layer which is grown on the underlying layer does not deteriorate.
Specific solutions will be listed below.
The first method for fabricating a nitride semiconductor device according to the present invention is the first solution, and includes the steps of: forming a semiconductor layer made of a first nitride semiconductor on a substrate; forming, on the semiconductor layer, a flatness maintenance layer made of a second nitride semiconductor for maintaining a surface flatness of the semiconductor layer by suppressing evaporation of constituent atoms of the semiconductor layer; and forming an active layer made of a third nitride semiconductor on the flatness maintenance layer.
According to the first method for fabricating a nitride semiconductor device, the flatness maintenance layer for maintaining the surface flatness of the semiconductor layer by suppressing evaporation of the constituent atoms of the semiconductor layer is formed on the semiconductor layer to be the underlying layer for the active layer before the active layer is formed. Thus, when the active layer is formed on the flatness maintenance layer, the crystallinity of the active layer is maintained at a high level. As a result, it is possible to improve the operating characteristics of the semiconductor device and to ensure the long-term reliability thereof.
In the first method for fabricating a nitride semiconductor device, it is preferred that the second nitride semiconductor includes aluminum, and the third nitride semiconductor includes gallium and indium. In this way, since the vapor pressure of aluminum nitride (AlN) is lower than that of gallium nitride (GaN), as will be described later, the constituent atoms of the flatness maintenance layer are unlikely to be evaporated. As a result, it is possible to reliably protect the semiconductor layer as the underlying layer. Moreover, since the third nitride semiconductor forming the active layer includes indium (In), the energy band gap is narrowed, and it is possible to reliably form a quantum well structure in the third nitride semiconductor
The second method for fabricating a nitride semiconductor device according to the present invention is the first solution, and includes the steps of: forming a first semiconductor layer made of a first nitride semiconductor on a substrate at a first growth temperature; forming, on the first semiconductor layer, a flatness maintenance layer made of a second nitride semiconductor for maintaining a surface flatness of the first semiconductor layer by suppressing evaporation of constituent atoms of the first semiconductor layer; and forming a second semiconductor layer made of a third nitride semiconductor on the flatness maintenance layer at a second temperature lower than the first growth temperature.
According to the second method for fabricating a nitride semiconductor device, the flatness maintenance layer for maintaining the surface flatness of the first semiconductor layer is formed on the first semiconductor layer at the first growth temperature, whereby it is possible to suppress evaporation of the constituent atoms of the first semiconductor layer from the surface thereof while the temperature is decreased from the first temperature to the second temperature. Thus, the crystallinity and flatness of the first semiconductor layer are maintained. Therefore, when the second semiconductor layer is formed on the first semiconductor layer, the crystallinity of the second semiconductor layer is maintained at a high level. As a result, assuming, for example, that the second semiconductor layer is the active region, the crystallinity of the active region is maintained at a high level, whereby it is possible to improve the operating characteristics of the semiconductor device and to ensure the long-term reliability thereof.
In the first or second method for fabricating a nitride semiconductor device, it is preferred that the second nitride semiconductor is made of AlxGa1xe2x88x92xN (where x satisfies 0.1 less than xxe2x89xa61). In this way, re-evaporation of the constituent atoms from the second nitride semiconductor to be the flatness maintenance layer does not occur, whereby it is possible to reliably suppress evaporation of the constituent atoms of the semiconductor layer made of the first nitride semiconductor and formed under the flatness maintenance layer.
In the first or second method for fabricating a nitride semiconductor device, it is preferred that the second nitride semiconductor is made of AlxGa1xe2x88x92xN (where x satisfies 0.2xe2x89xa6xxe2x89xa61). In this way, it is possible to more reliably suppress evaporation of the constituent atoms of the semiconductor layer made of the first nitride semiconductor and formed under the flatness maintenance layer.
In the first or second method for fabricating a nitride semiconductor device, it is preferred that the thickness of the flatness maintenance layer is approximately 0.5 xcexcm or less. In this way, a crack does not occur in the flatness maintenance layer.
The third method for fabricating a nitride semiconductor device according to the present invention is the second solution, and includes: a first semiconductor layer formation step of introducing a group III material gas and a group V material gas onto a substrate, thereby forming a first semiconductor layer made of a first nitride semiconductor on the substrate; a group III material gas stopping step of stopping the introduction of the group III material gas; and a second semiconductor layer formation step of introducing a group III material gas and a group V material gas onto the first semiconductor layer, thereby forming a second semiconductor layer made of a second nitride semiconductor on the first semiconductor layer, wherein the group III material gas stopping step includes the step of adjusting an ambience around the first semiconductor layer to be an ambience containing nitrogen and having a pressure of 1 atm or higher.
According to the third method for fabricating a nitride semiconductor device, it is possible to suppress evaporation of the constituent atoms of the first semiconductor layer, particularly nitrogen atoms, from the first semiconductor layer. Thus, the crystallinity and flatness of the first semiconductor layer are maintained. Therefore, when the second semiconductor layer is formed on the first semiconductor layer, the crystallinity of the second semiconductor layer is maintained at a high level. As a result, assuming, for example, that the second semiconductor layer is the active region, the crystallinity of the active region is maintained at a high level, whereby it is possible to improve the operating characteristics of the semiconductor device and to ensure the long-term reliability thereof.
The fourth method for fabricating a nitride semiconductor device according to the present invention is the second solution, and includes: a first semiconductor layer formation step of introducing a group III material gas and a group V material gas onto a substrate along with a first carrier gas, thereby forming a first semiconductor layer made of a first nitride semiconductor on the substrate; a carrier gas changing step of changing the first carrier gas to a second carrier gas whose composition is different from that of the first carrier gas; and a second semiconductor layer formation step of introducing a group III material gas and a group V material gas onto the first semiconductor layer along with the second carrier gas, thereby forming a second semiconductor layer made of a second nitride semiconductor on the first semiconductor layer, wherein the carrier gas changing step includes the step of adjusting an ambience around the first semiconductor layer to be an ambience containing nitrogen and having a pressure of 1 atm or higher.
According to the fourth method for fabricating a nitride semiconductor device, it is possible to suppress evaporation of the constituent atoms of the first semiconductor layer, particularly nitrogen atoms, from the first semiconductor layer. Thus, the effects as those of the third method for fabricating a nitride semiconductor device can be obtained.